This application is based upon and claims the benefit of priority of Japanese Patent Applications No. 2000-340655 filed on Nov. 8, 2000 and No. 2001-130123 filed on Apr. 26, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a load detecting device for detecting load applied to a mechanical part, for example, the load applied to a brake pedal when a driver depresses the brake pedal or the load applied to a vehicle body when the car collides.
2. Description of Related Art
A conventional load detecting device has a cylinder, a piston housed in the cylinder, liquid with which inside space between the cylinder and the piston is filled up and a sensor element. When load is inputted to a piston from an outside mechanical part, pressure of the liquid in the cylinder increases according to the movement of the piston. The sensor element senses the pressure of liquid so that the load inputted to the piston is detected.
However, the conventional load detecting device has a drawback that a detecting accuracy is low since there exists a hysteresis of piston movement due to sliding friction between the piston and the cylinder. Further, it is very difficult to completely prevent liquid leakage with a seal between the piston and cylinder, while a life time endurance of the seal is not so long. The liquid leakage causes the piston to shift, resulting in poor detecting accuracy.
Moreover, when eccentric load, which presses the piston not axially but obliquely, is applied to the piston, the piston is likely to incline so that the sliding friction between the piston and the cylinder becomes larger. As a result, the detecting accuracy of the load detecting device is lower.
An object of the invention is to provide a load detecting device with higher detecting accuracy.
To achieve the above object, a load detecting device has a bellows unit which expands and contracts axially according to a load applied to an axial end thereof. The bellows unit is provided inside with a sensor chamber which is hermetically sealed from outside and whose pressure is variable according to an expanding and contracting movement thereof. A pressure detecting element is installed in the sensor chamber. With this construction, the pressure detecting element detects the pressure of the sensor chamber and generates a signal representing the load applied to the bellows unit.
As the pressure of the sensor chamber inside the bellows unit varies according to the load applied thereto, the load applied to the load detecting device can be detected by sensing the pressure of the sensor chamber with the pressure detecting element.
As mentioned above, the seal between the piston and the cylinder, as the conventional load detecting device has, is not necessary and there is no hysteresis caused by sliding friction due to the seal. Accordingly, the load detecting device, which does not have the seal and the hystresis based on the seal, has a longer life time endurance with higher detecting accuracy.
It is preferable that the sensor chamber is filled up with liquid so that the pressure detecting element detects pressure of the liquid. As a volume change of the liquid due to temperature change can be absorbed by the expansion and contraction of the bellows unit, the load detecting device has better temperature characteristic.
Preferably, the bellows unit is composed of a bellows which is formed in cylindrical shape with hollow and expands and contracts axially, a sensor head fixed fluid-tightly to an axial end opening of the bellows, and a base housing fixed fluid-tightly to an axial another end opening of the bellows. In this case, the pressure detecting element is installed on the base housing and the load is applied to the bellows via the sensor head.
It is preferable that the base housing is composed of a housing, whose outer circumference is fixed to the axial another end opening of the bellows, is provided with a hollow and a stem inserted into the hollow and fixed to the housing so that a part of the hollow remains in the housing on a side of the bellows. In this case, the pressure detecting element is installed on the stem in the remaining part of the hollow.
As an alternative, the bellows unit may be composed of a cup shaped bellows whose cross sectional shape is one side opened square and which expands and contracts axially, a housing fixed fluid-tightly to an opening of the bellows and a sensor head fixed to a bottom of the bellows. In this case, the load is applied to the bellows via the sensor head and the pressure detecting element is installed on the housing.
It is preferable that the sensor head or the housing is provided with a through-hole connecting between an inside of the sensor chamber and an outside thereof and a lid disposed in the through-hole for fluid-tightly sealing the through hole. After the liquid is poured into the sensor chamber, the lid is put in the through-hole so that the manufacturing of the load detecting device is easier.
Further, it is preferable that the sensor head or the base housing has a portion protruding into the sensor chamber beyond a position where the sensor head or the base housing is fixed to the bellows. With this construction, the volume of the sensor chamber becomes smaller so that the volume of the liquid is smaller. Accordingly, since the volume change of the liquid due to temperature change is limited, the load detecting device has better temperature characteristic. In addition to the advantage mentioned above, the protruding portion of the sensor head or the housing serves to prevent the bellows from excessively inclining when the bellows expands and contracts, resulting in a longer life time endurance of the bellows.
Furthermore, it is preferable that the load detecting device has a floating load transmission member having first and second covers coupled axially slidably with each other and a spring disposed between the first and second covers. With the floating load transmission member, the second cover is fixed to the sensor head so that the load, which is applied to the first cover, is transmitted to the bellows via the spring, the second cover and the sensor head.
Moreover, the pressure detecting element is made of, preferably, semiconductor substrate and is provided with a diaphragm portion formed by partly thinning the semiconductor substrate, an element portion for generating an output signal corresponding to pressure acting on the diaphragm portion and a process circuit portion for processing the output signal of the element portion. Since the element portion and the process circuit portion are integrated and housed in the sensor chamber, the distance between the process circuit portion and the element portion is shorter so that the load detecting device is unlikely influenced by radio noises.